The present invention relates to line printers and more particularly to novel open-loop D.C. motor control apparatus for providing essentially constant printing speed thereof.
It is well known that the quality of printing in an electronic dot-matrix impact printer depends largely upon the magnitude of speed variations as the print head moves across the printing medium. The degree of registration of the printed dots forming each character, and hence the overall print quality, is reduced due either to variations in speed during printing or by initial print head acceleration to a speed beyond the desired constant speed. In a typical impact printer, the carriage driving motor is continually operated directly from the main printer power source, without the possibility of adjusting the motor voltage and hence the motor output speed. The continuously energized motor is coupled to the printer carriage via a speed reducer, a pair of clutches energizable in mutually exclusive fashion for selecting movement in the forward or the reverse directions, and a brake mechanism energizable to remove the driving torque from the carriage and rapidly overcome its inertia thereby halting the carriage. Such a carriage drive system is not only bulky but also requires a large number of costly components. Additionally, there is no manner in which the desired constant printing speed may be easily obtained, nor is any system adjustment provided to prevent carriage speed overshoot when the carriage is initially accelerated in either direction upon engagement of either the forward or the reverse clutch mechanism.
In a line printer adapted to print during carriage travel in both the forward and reverse directions, each change of direction of travel requires that the energized clutch be decoupled from the motor and the brake mechanism be energized to substantially halt carriage motion to allow the remaining clutch to be energized against a relatively low reverse torque. The relatively long actuation time intervals of the electro-mechanical mechanisms required by this interrelationship significantly reduces useful printer speed as a significant portion of total carriage travel time is used solely for direction reversal.
In a line printer adapted to print during carriage travel only in the forward direction, increased printer performance requires that the carriage not only be smoothly accelerated to a constant printing speed in the forward direction but also that the duration of travel in the reverse direction be as short as practical. One suggestion for achieving rapid reverse travel comprises additional gearing means in the reverse clutch mechanism to more rapidly accelerate the carriage in the reverse direction, thereby requiring a shorter travel time interval. This solution is undesirable as requiring additional costly mechanical components and is especially undesirable in a printer of the unidirectional-printing type adapted for a return operation after printing a variable portion of a line of characters in the forward direction, as the carriage may be damaged if increased reverse acceleration and return speed are not controlled in proportion to the length of line printed.
In either line printer type, a braking operation to temporarily halt the motion of the carriage at any time and at any point along the line may still be required.
It is desirable to provide a line printer with a motor control means allowing removal of the forward and the reverse motion clutches and the separate braking mechanism, while providing rapid acceleration to printing speed without overshoot and then maintaining essentially constant print head speed while the carriage-mounted printed head traverses the width of the printing medium. It is also desirable to provide a motor control means allowing rapid acceleration to printing speed in an opposite direction; braking capability at any point along its path of travel; and in a unidirectional type printer, increased acceleration in the reverse direction.